The invention relates to a method of manufacturing micro-components having at least one individual layer, which can be used in the chemical industry inter alia for synthesis reactions and in other fields, for example as reactors for generating hydrogen for energy conversion (fuel cells), as well as heat exchangers, mixers and evaporators.
In the literature, there have been reports for some years now of chemical microreactors which have advantages in comparison with the traditional production plants for manufacturing chemical compounds. Here it is a question of an arrangement of reaction cells, the dimensions of which range between a few micrometers and several millimeters, and thus are very much smaller than the traditional reactors. These reaction cells are so designed that in them physical, chemical or electrochemical reactions can take place. In contrast to a traditional porous system (heterogeneous catalysis), the dimensions of these cells are defined by their construction, therefore may be manufactured systematically by means of a technical method. The arrangement of individual reaction cells in the assembly of the reactor is ordered, particularly periodically in one, two or three dimensions. Amongst the chemical microreactors are counted, in a wider sense, also the necessary supply and discharge structures for the fluids (liquids, gases) and sensors and actuators, for example valves which control the flow of substance through the individual cells, and heating elements.
The use of chemical microreactors for generating hydrogen for fuel cells for the conversion of energy has been described for example by R. Peters et al. in xe2x80x9cScouting Study about the Use of Microreactors for Gas Supply in a PEM-Fuel Cell System for Tractionxe2x80x9d, Proc. of the 1st Int. Conf. on Microreaction Technology, Frankfurt, 1997.
This concept for chemical microreactors has also been applied to heat exchangers. In this case, at least two fluid channels separate from one another are present in the heat exchanger and serve to transfer heat from fluid in the one channel to fluid in the other channel.
There is a series of proposals for the manufacture of chemical microreactors or heat exchangers:
For example the LIGA-process (lithography, electroforming, moulding) is used. In this process a plastics material layer, mostly polymethyl methacrylate (PMMA) is exposed to synchrotron radiation and then developed. The structure produced in this way is filled with metal by means of an electrolytic process. The metal structure can then be multiplied in further method steps by means of plastics moulding (plastics injection moulding). This method was described by W. Ehrfeld and H. Lehr in Radiat. Phys. Chem., Vol. 45, pages 349 to 365.
The methods which have been developed in the semiconductor industry for structuring silicon surfaces have also been adopted for manufacturing of microreactors. For instance in xe2x80x9cMicrofabricated. Minichemical Systems: Technical Feasibilityxe2x80x9d, DECHEMA Monographs, Volume 132, pages 51 to 69 a method is described by J. J. Lerou et al. in which three etched silicon wafers and two end wafers are connected to one another on the outer sides. Furthermore, a heat exchanger filled with polycrystalline silver particles is used which was also configured as a microreactor.
In the same way, the method of manufacturing microreactors which is described in U.S. Pat. No. 5,534,328 also proceeds from etched silicone wafers which are joined into a stack. However, other materials are also mentioned for the microreactors, for example metals, polymers, ceramics, glass and composite materials. To carry out catalytic reactions it is proposed inter alia that the walls of the reaction channels in the reactors be coated with a catalytic layer.
In EP 0 212 878 A1, a method of manufacturing a heat exchanger is described in which the flow channels of the heat medium are formed in steel plates by chemical etching. The steel plates are then welded to one another by diffusion bonding.
In WO-A-9215408, a method of manufacturing microstrainers is described in which perforations are etched in a certain pattern through plasma technology into a flat carrier coated with an etch-resistant layer. Several of these perforated carriers are then connected to one another.
In DE 197 08 472 A1, a method of manufacturing chemical microreactors is described in which fluid channels are formed in individual planes, by substrates provided with metal surfaces being structured by means of photolithographic techniques or screen printing methods, and the channel structures obtained being formed by methods of removing or depositing metal. The individually produced planes are then combined to form a stack and securely connected to one another. For example, the channels can be produced by partial etching away of the metal layer on the substrate.
The previously known methods for manufacturing chemical microreactors and heat exchangers have manifold disadvantages. For example complicated and/or expensive techniques are necessary for producing the channels. In some cases, the manufacture of reactors is limited exclusively to silicone as a material.
It is frequently also necessary to produce a functional coating on the channel walls to set pre-determined properties of the micro-components. Thus for example a microreactor can be produced from a heat exchanger manufactured from copper, by the channels being coated with a metal layer deposited in an electroless manner, for example with palladium. In chemical reaction technology, the functional surface layers serve for example the catalysis of chemical reactions. Subsequent coating of the flow channels in the planes by means of a galvano-technical method is however frequently not possible since the functional layers in this case cannot be applied electrolytically on account of the electrical shielding by the reactor or heat exchanger itself. In electroless metallisation, also, it has emerged that secure coating is not possible since the metallisation baths usually used react very sensitively to different flow speeds of the metallisation fluid on the surfaces to be coated. Under these conditions, inter alia those surface regions past which the metallisation fluid flows slowly are metallised in an electroless manner whilst surface regions past which the fluid flows at a high speed are not coated with metal. With very narrow channels, problems can occur in electroless metal deposition which is based on the very high bath load (surface to be coated per bath volume), such that only inadequate layer qualities are produced. Possibly a layer formation with total covering of the surface becomes completely impossible. Moreover by means of electroless methods only certain metals can be deposited.
Gas deposition methods for applying layers are in this case practically unusable.
In the cases in which the functional layers are applied before the individual layers are joined to form the micro-component, the connection of the individual component layers has proved to be problematic since no reliable connection can be produced between the individual layers. Frequently the components produced from the individual layers have leaks from which fluid which is under relatively high pressure penetrates outwards from the channels.
Furthermore the functional layers are not stable in relation to the joining temperatures usually used to join together the individual layers. The functional layers, particularly in cases in which the material has a lower melting or transition temperature than the temperature during joining, are damaged or even destroyed. In particular also noble metals such as platinum, iridium, palladium and gold can be applied to form the functional layers. These metals admittedly have a higher melting point than the copper usually used as a basic material for the micro-component and should therefore be thermally resistant in a joining process in which the basic materials of two individual layers are welded to one another. However, in this and other similarly layered cases, it has been noticed that for example in diffusion bonding micro-structured reactor foils coated in this way, such high bonding temperatures are necessary that the functional layer and the basic material become mixed with one another even before there is sufficient connection of the basic materials to one another. Thus the functionality of the layer is destroyed, for example a palladium layer on an individual layer consisting of copper, since palladium diffuses very quickly into the copper under these conditions.
For these reasons, manufacturing methods in which the functional layers are applied before the individual layers are joined together, have not been considered practicable.
The problem underlying the present invention, therefore, is to manufacture micro-components having at least one individual layer, which have inner structures delimited by walls, for example flow channels, in the components and functional layers on the walls of the structures. The micro-components should be suitable for a large number of different applications in chemical reaction technology, for heat exchanging, for mixing substances or for evaporating liquids. In particular it should be possible to apply different coatings to the channel surfaces for different applications of the micro-component. Furthermore, the method of manufacturing should be able to be carried out as quickly and as cheaply as possible without high failure rates occurring in the production of the micro-components. Microreactors, heat exchangers, mixers and evaporators of this kind should also be able to be manufactured in large numbers simply and cheaply. In particular it should be possible to produce micro-components which have no problems in respect of leaks from the flow channels, and in which no problems occur in respect of the stability of the functional layers when the individual layers are joined.
This problem is solved by the method according to Claim 1. Preferred embodiments of the invention are quoted in the subordinate claims.
In particular the problem that the known manufacturing methods do not succeed in producing microreactors and other micro-components which are completely free of leaks at the joining points, can be solved by the method according to the invention. Only by extensive experiments was it possible to explain the cause of the leaks by the fact that the functional layers frequently exercise a harmful influence on the cohesion of the layers to one another. A solution to this problem has proved to be the functional layers being formed exclusively on the walls of the channels before the individual layers are stacked up and joined to one another and to a segment terminating the flow channels. What is namely thereby avoided is that functional layers are also formed at the locations on the micro-structured layers which are necessary for joining the layers together.
The method according to the invention also offers the advantage that the joining process can be carried out at a low temperature. By this means, micro-components with temperature-sensitive functional layers can also be produced without these layers being impaired. Since under these conditions, a diffusion of the functional layers into the basic material of the first metal layer or metal foil can be largely prevented, thermally sensitive functional layers can also be used.
Through the method, also, the problems do not appear which occur if the functional layers are only applied after the individual layers have been joined together to form the micro-component. In particular, gas deposition methods can also be used without problem to apply layers, for example to form layers which cannot be produced with an electroless metal deposition method. Furthermore, electroless or electrolytic metallisation is also readily successful.
The method according to the invention serving to manufacture the micro-components consists of the following sequence of method steps:
A. producing an individual layer by
a. producing a first metal layer or a metal foil;
b. forming the inner structures in and/or on the first metal layer or metal foil, for example the flow channels, by suitable etching methods and/or metal deposition methods;
c. forming the functional layers solely on the walls of the inner structure;
B. thereafter stacking and joining the individual layer to a segment terminating the flow channels or a plurality of individual layers to one another and to the terminating segment.
The flow channels in and/or on the first metal layer or metal foil are formed in one embodiment of the invention preferably by the following method steps:
b. copying the flow channels on the first metal layer or metal foil by a structuring method; and
bxe2x80x2. selective etching of the regions of the first metal layer or metal foil which are exposed during structuring.
As copying methods, the following method alternatives can be used:
b1. coating at least one surface of the first metal layer or metal foil with a photosensitive layer, for example a photoresist usually used in printed circuit board technology or semi-conductor technology, exposing the photosensitive layer with the pattern of the flow channels and exposing the first metal layer or metal foil at all the locations which correspond to the channels to be formed;
b2. coating at least one surface of the first metal layer or metal foil with a screen-printing. lacquer layer at the locations on the surface which do not correspond to the channels to be formed;
b3. laminating a perforated foil, for example a plastics film, onto at least one surface of the first metal layer or metal foil, the perforations being provided at all the locations of the surface which correspond to the channels to be formed.
In principle a metal resist can also be used, i.e. a second metal layer which is applied to the first metal layer or the metal foil, and which is structured by means of one of the methods described above. To this end, one of the previously mentioned covering layers or respectively the perforated foil is applied to the metal resist layer, structured itself and perforations are introduced in the metal resist layer at the locations at which the metal resist layer is exposed. Thereafter, the covering layer or the perforated foil is removed again before further processing.
The photosensitive layer, the screen-printing lacquer layer or the perforated foil can be removed again between the method steps c (forming the functional layers solely on the walls of the flow channels) and B (stacking and joining together the individual layers and the terminating segment) or after method step B. In a further embodiment of the invention, additional metal layers can also be applied, before the individual layers are joined together, solely on the surface regions of the first metal layer or metal foil which are not coated with the functional layers, by the following method steps being carried out:
d. coating solely the functional layers by a protective film which is soluble in a different solvent from the photosensitive layer, the screen-printing lacquer layer or the perforated foil;
e. forming additional metal layers after removing the photosensitive layer, the screen-printing lacquer layer or the perforated foil on the bare surface regions of the first metal layer or metal foil; and
f. removing the protective film.
With this method variant, for example metal layers can be applied to the surface regions of the first metal layer or metal foil, which serve as solder layers when the-individual layers are joined together. For example a tin/lead or a tin/bismuth alloy layer can be deposited.
In a further variant of the last mentioned method, instead of the additional metal layer also an adhesive layer can be applied solely to the surface regions of the first metal layer or metal foil which are not coated with the functional layers, by
d. only the functional layers being covered by a protective film;
e. the adhesive layers being formed after removing the photosensitive layer, the screen-printing lacquer layer or the perforated foil on the bare surface regions of the first metal layer or metal foil; and
f. the protective film being removed again.
Adhesive layers serve just like the previously mentioned solder layers to join together the individual layers, the adhesive layers being used in particular if particularly temperature-sensitive functional layers are formed in the flow channels, such that an increased temperature may not be set during the joining process.
In a further advantageous embodiment of the invention, the flow channels, provided with functional layers, are formed in the first metal layer or metal foil by the following sequence of method steps:
c. forming a functional layer on the first metal layer or metal foil;
b. copying the flow channels by one of the following method alternatives:
b1. coating at least one surface of the functional layer with a photosensitive layer, exposing the photosensitive layer with the pattern of the flow channels and exposing the first metal layer or metal foil, coated with the functional layer, at all the locations which correspond to the channels to be formed; or
b2. coating at least one surface of the functional layer with a screen-printing lacquer layer at the locations on the surface which do not correspond to the channels to be formed; or
b3. laminating a perforated foil onto at least one surface of the functional layer, the perforations being provided at all the locations of the surface which correspond to the channels to be formed; and
bxe2x80x2. forming depressions in the functional layer and possibly in the first metal layer or metal foil by selective etching of the bare regions of the functional layer and possibly of the first metal layer or metal foil;
bxe2x80x3. forming metal webs exclusively in the depressions formed in method step bxe2x80x2.
In the case where the photosensitive layer, the screen-printing lacquer layer or the perforated foil protrudes beyond the metal webs formed in the depressions, by grinding and/or polishing a substantially flat surface can be produced formed by the metal webs and the photosensitive layer, the screen-printing lacquer layer or the perforated foil. In this case too, the photosensitive layer, the screen-printing lacquer layer or the perforated foil is removed again before the individual layers are joined together. The webs form the walls of the flow channels.
After the individual layers have been produced by one of the method alternatives described above, the micro-component is produced by stacking and joining the individual layers to one another and to the terminating segment. For this purpose, according to the technology used, a soldering or welding method, for example a diffusion bonding method, or a gluing method can be used.
The first metal layer or metal foil consists preferably of copper, steel or aluminium.